Fix missing assertions for native access functions

src/wgl_context.c

@@ -776,6 +776,8 @@ GLFWbool _glfwCreateContextWGL(_GLFWwindow* window,
 GLFWAPI HGLRC glfwGetWGLContext(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
 
     if (_glfw.platform.platformID != GLFW_PLATFORM_WIN32)